This invention relates to electromagnetic wave absorbing, heat conductive silicone rubber compositions suitable for forming heat dissipating sheets disposed between electronic equipment components (such as Central Processing Unit (xe2x80x9cCPUxe2x80x9d), Microprocessing Unit (xe2x80x9cMPUxe2x80x9d), and Large Scale Integration (xe2x80x9cLSIxe2x80x9d)) and heat sinks for conducting heat to the heat sinks while suppressing the noise originating from the electronic equipment components.
Since electronic equipment components such as CPU, MPU and LSI generate heat during operation, heat dissipating elements including heat dissipating media such as silicone grease and silicone rubber and metallic heat sinks are frequently used for cooling the electronic equipment components.
As the degree of integration of CPU is increased to meet the demand for size reduction, the amount of heat release per unit area is increased. Ineffective cooling will cause thermal runaway or undesired effects, giving rise to the problem of CPU malfunction. While it is desired to increase the operating frequency of CPU in order to meet the demand for high speed processing, a high frequency disturbance or noise is concomitantly produced which will join signals on communication lines, causing detrimental effects including malfunction.
One typical means for effectively radiating heat from CPU is to use heat dissipating media such as silicone grease and silicone rubber for effectively transferring heat from CPU to metallic heat sinks. This means, however, cannot avoid the problem of malfunction by noise because the silicone rubber and other heat dissipating media are not effective for absorbing electromagnetic waves or suppressing noise.
A noise suppression effect is achieved by adding ferrite materials such as Mnxe2x80x94Zn ferrite and Nixe2x80x94Zn ferrite to silicone gel as disclosed in JP-A 11-335472. The noise which can be suppressed by this means is sometimes limited to the low frequency region. Problems of reliability and long-term storage stability arise since the ferrite materials are liable to rust.
An object of the invention is to provide an electromagnetic wave absorbing, heat conductive silicone rubber composition suitable for forming heat dissipating sheets having a heat dissipating function while exhibiting a noise suppression function over a wide frequency region from more than 10 MHz to less than 100 GHz, especially the quasi-microwave region.
It has been found that by incorporating in silicone rubber a soft magnetic metal powder, especially iron or an iron alloy, preferably in combination with a heat conductive filler (exclusive of the soft magnetic metal powder), there is obtained an electromagnetic wave absorbing, heat conductive silicone rubber composition suitable for forming heat dissipating sheets capable of simultaneously suppressing noise generation from electronic equipment components such as CPU, MPU and LSI, and transferring heat from the same.
The invention provides an electromagnetic wave absorbing, heat conductive silicone rubber composition comprising a soft magnetic metal powder and having in the cured state a thermal conductivity of at least 2.0 w/mK. The soft magnetic metal powder is typically selected from iron and iron alloys. The composition may further contain a heat conductive filler.